1. Field of the Invention
This invention relates to a process for preparing a silicone rubber composition that can produce a silicone rubber showing superior compression set properties.
2. Description of the Prior Art
Silicone rubber is widely used in the field of electronic equipment, automobiles, construction, medical treatment and so forth because of its good weatherability, electrical properties, freeze resistance, and compression set properties (resistance). In particular, silicone rubber is used in O-rings, gaskets, sealing mediums and roll materials because of its good compression set properties retainable at a wide temperature range and over a long period of time.
In recent years, a variety of methods for further improving the compression set properties of silicone rubber have been studied. For example, methods are known in which metal oxides such as cerium oxide are mixed, or unsaturated groups contributing to cross-linking are introduced to the side chains or terminals of organopolysiloxanes. Such methods, however, only enable decrease of compression set to a certain extent and have the a disadvantage that other physical properties deteriorate.
Japanese Unexamined Patent Publication (Kokai) No. 3-277658 discloses a method in which a silicone elastomer base material containing a silanol group, having been heated, is mixed with a disilazane containing an alkenyl group (an aliphatic unsaturated hydrocarbon group), followed by heating. This method can achieve a lower compression set, but has a difficulty of making other physical properties poor, e.g., causing an increase in hardness and a decrease in elongation and tear strength.